Phosphotungstic acid supported on magnetic nanoparticle used as selective and reusable photocatalyst for aerobic oxidation of benzylic alcohols

Article abstract of DOI:10.13005/ojc/290457

An efficient and selective photochemical method for the oxidation of benzylic alcohols has been achieved using phosphotungstic acid supported on imidazole functionalized silica coated cobalt ferrite nanoparticles as a magnetically photocatalyst in acetonitrile with O₂. The photocatalyst has been reused several times, without observable loss of activity and selectivity. Results showed this heterogeneous photocatalyst is more active than the unsupported H₃PW₁₂O₄₀.

Full text of DOI:10.13005/ojc/290457

ISSN: 0970-020 X
CODEN: OJCHEG
013, Vol. 29, No. (4):
Pg. 1675-1681
ORIENTAL JOURNAL OF CHEMISTRY
An International Open Free Access, Peer Reviewed Research Journal
2
www.orientjchem.org
Phosphotungstic Acid Supported on Magnetic
Nanoparticle Used As Selective and Reusable
Photocatalyst for Aerobic Oxidation of Benzylic Alcohols
1
1
2
MOꢀHꢁAN AFSHARI *, MARYAM ꢁORJIꢀADEH and ꢁHAꢀAL AFSHAR
1
Department of Chemistry, Shoushtar Branch, Islamic Azad University, Shoushtar, Iran.
2
Department of Chemistry, Science and Research Branch, Islamic Azad University, Khouzestan, Iran.
http://dx.doi.org/10.13005/ojc/290457
(Received: November 04, 2013; Accepted: December 10, 2013)
ABSTRACT
An efficient and selective photochemical method for the oxidation of benzylic alcohols has
been achieved using phosphotungstic acid supported on imidazole functionalized silica coated cobalt
ferrite nanoparticles as a magnetically photocatalyst in acetonitrile with O . The photocatalyst has
2
been reused several times, without observable loss of activity and selectivity. Results showed this
heterogeneous photocatalyst is more active than the unsupported H PW O
3
12 40
Key words: Cobalt ferrite, Oxidation, Magnetic nanoparticles, Surface functionalization, Phosphotungstic acid.
INTRODUCTION
Despite of some advantages, owing to the good
solubility of POMs in polar solvents heterogenization
of the homogeneous catalysts is very attractive
to overcome the difficulty of recovery. A possible
method is immobilizing POMs species in the surface
of magnetic particles which can be separated and
recovered from the reaction system by applying an
The oxidation of alcohols into their
corresponding carbonyl compounds represents
a fundamentally important functional group
transformation and occupies a prominent position
in modern synthetic organic chemistry . Many
thermal catalytic systems have been designed and
developed for the oxidation of alcohols . Most of
these systems are particularly effective for activated
benzylic and allylic) alcohols.However, the search for
suitable green catalysts is still a significant challenge.
In this context, photoreactions are promising
processes and the development of photocatalysts
is a subject that is now receiving much attention. An
important class of photocatalysts that has received
1
15–25
appropriate magnetic field
. Moreover compared
2-13
to conventional supported POMs photocatalysts,
the nanometer sizes of the magnetic photocatalyst
provide a larger surface area, which subsequently
enables higher efficiency in photo-catalytic reactions.
However, to the best of our knowledge, only a
few reports have appeared in the literature about
photocatalytic activity of polyoxometalates supported
on MNPs. The first example of immobilization of
POMs on MNPs, as efficient photocatalysts for
(
14
more attention is the polyoxometalates (POMs)
.

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AFSHARI et al., Orient. J. Chem., Vol. 29(4), 1675-1681 (2013)
degrading Rhodamine B was reported by Zheng decanted solution was purified on a silica-gel plate
2
6
et.al ; they prepared magnetic photocatalyst by or a silica-gel column to obtain the pure product.The
grafting polyoxometalates (POM) anions onto Fe O4 identities of the products were confirmed by FT-IR
3
1
nanoparticles via a layer of Ag. In another report, and H NMR spectral data.
Li et.al fabricates the amino functionalized magnetic
core–shell mesoporous microspheres, which play
as the support for immobilizing H PW O onto the
RESULTS AND DISCUSION
3
12 40
surface of the microspheres. This composite also Characterization OF PTA/Si-imid@ Si-MNPs
shows high efficiency in decomposing Rhodamine
B under UV irradiation .
The magnetic core was analyzed by XRD
27
and the peaks are compatible with pure CoFe O4
2
phase (JCPDS PDF #221086), indicating the
In the present work, the phosphotungstic retention of cubic reverse spinel structure of CoFe O4
2
acid supported on imidazole functionalized silica during coating and functionalization (Fig. 1).
coated cobalt ferrite nanoparticles (PTA/Si-imid@
Si-MNPs) was used, for the first time, as an
The magnetic properties of the silica coated
efficient and magnetically recyclable heterogeneous cobalt ferrite (Si-MNPs)and PTA/Si-imid @ Si-MNPs
photocatalyst for aerobic oxidation of various were summarized in Table 1. The decrease in mass
primary and secondary benzylic alcohols into the saturation magnetization may be ascribed to the
corresponding carbonyl compounds.
contribution of the non-magnetic silica shell and
functionalized groups
EXPERIMENTAL
ThepresencePTAwasconfirmedbyIR(see
Fig. 2). The TEM micrograph of the nanoparticles
ꢁeneral
All chemicals were purchased from is shown in Fig.3.TEM image indicated that most of
Sigma-Aldrich or Merck and used as received. the prepared nanoparticles are spherical shaped and
Phosphotungstic acid supported on magnetic have size less than 20-30 nm.Moreover, the tungsten
nanoparticles was prepared using the procedure content of the catalyst, as determined by ICP-AES,
18
reported by Kooti et al. .FT-IR spectra were obtained was 0.98 mmol/g
using BOMEM MB-Series 1998 FT-IR spectrometer.
Magnetic properties of all nanoparticles were Alcohols
measured with a vibrating sample magnetometer
In order to optimize the reaction conditions
(
VSM, Meghnatis Daghigh Kavir Coumpany) at for the oxidation of benzylic alcohols various
room temperature. NMR spectra were recorded experiments were carried out at room temperature
in CDCl on a Bruker Advanced DPX 400 MHz using 1-phenylethanol in the presence of PTA/Si-
3
spectrometer.
imid@ Si-MNPs as photocatalyst under O2.
Catalytic studies
To optimize the catalyst requirement, the
In a Pyrex flask equipped with a magnet amount of catalyst was changed between 20 and 150
bar, a solution of the alcohol (5 mmol) in acetonitrile
(
10 mL) was prepared.To this solution PTA/Si-imid@
Table 1. Magnetic property of
the as-fabricated nanoparticles
Si-MNPs (50 mg) was added and purged with O and
2
joined through an inlet tube to a balloon filled with O2
before irradiation.
compound
Hc
Mr
Ms
(
Oe)
(emu/g)
(emu/g)
The suspension was vigorously stirred and
irradiated with a high pressure 400 W mercury lamp MNPs
using a cut-off Pyrex filter at room temperature.The Si-MNPs
progress of the reaction was monitored by TLC. At PTA/Si-imid @
the end of the reaction the organic layer was simply Si-MNPs
decanted by means of an external magnet. The
796.1
861.05
24.63
16.40
59.49
38.93
688.05
6.54
21.84

AFSHARI et al., Orient. J. Chem., Vol. 29(4), 1675-1681 (2013)
1677
Table 2. Oxidation of alkenes PTA/Si-imid@ Si-MNPs photocatalyst a
b
Entry Alochol
Produt
Time (h)
Yield %
OH
O
1
1.5
82
O
HO
2
1
1
92
94
OH
O
3
O
HO
4
1
92
Cl
Cl
OH
O
5
6
7
1.5
1
81
92
91
OH
O
MeO
MeO
MeO
MeO
O
OH
0.5
OH
O
O
2
N
2
O N
8
9
3.5
1.5
65
66
O
OH
OH
OH
O
O
O
1
1
0
1
1
1
72
90
O
O
OH
CH
3
O
CH
3
O
CH
3
O
3
CH O
1
1
1
2
3
4
1
94
75
78
OH
O
O
S
O
S
15
1.5
OH
O
HO
O
OH
OH
15
1.5
85
O
HO
c
16
1.5
86
a
Reaction conditions: allcohol(5mmol), catalyst (50 mg), acetonitrile (10 ml), O atmosphere, rt.
2
bIsolated yields, ccatalyst : tenth recycled catalyst

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AFSHARI et al., Orient. J. Chem., Vol. 29(4), 1675-1681 (2013)
Fig. 1: XRD pattern of (a) MNPs, (b) Si-MNPs and (c) PTA/Si-imid@ Si-MNPs
Fig. 2: FT-IR spectrum of PTA/Si-imid@ Si-MNPs
Fig. 3:TEM image of PTA/Si-imid@ Si-MNPs

AFSHARI et al., Orient. J. Chem., Vol. 29(4), 1675-1681 (2013)
1679
Scheme 1: Oxidation of 1-phenylethanol using PTA/Si-imid@ Si-MNPs catalyst
subjected to 1 h irradiation in the presence of PTA/Si-
imid@ Si-MNPs under O . Result indicated that the
2
former was oxidized to acetophenone in 92% yield
while 2-phenylethanol was recovered completely.
Moreover, we have also examined the
photocatalytic oxidation of 2-phenylethanol under
pure O (1 atm), air and N . It was found that
2
2
the efficiency for the photocatalytic oxidation of
-phenylethanol depends on the concentration of O2,
2
the photocatalytic activity was higher in the presence
of pure O than in air, and almost no activity was
2
observed under N2.
At optimum conditions we observed that
when a heterogeneous mixture of this alcohol and
PTA/Si-imid@Si-MNPs was irradiated under O2
atmosphere in acetonitrile, acetophenone (2) was
formed as the only product in 92% yield within 1 h
at room temperature (see scheme1).
Fig. 4:TEM image of recycled
PTA/Si-imid@ Si-MNPs
On the other hand, 1 was oxidized to
acetophenone in a 72% yield within 2. 5h when pure
mg per 5 mmol 1-phenylethanol alcohol. Increase
H PW12O40 (PTA) was used as the photocatalyst
3
in the yield of acetophenone was observed when under the same reaction conditions. This result
the amount of catalyst was increased from 20 to 50 clearly indicated that immobilized H
mg but the yield remained almost same with further magnetic nanoparticles is more active than the bulk
PW12O40 on
3
increment of catalyst amount up to 150 mg. An PTA and PTA/SiO
.
2
optimum of 50 mg catalyst in the reaction mixture
is ideal for achieving the best yield. We have also
The obtained preliminary results for the
accomplished the oxidation of 1-phenylethanol in oxidation of 1-phenylethanol were encouraging and
the different solvents such as n-hexane, acetone, to extend the scope of this heterogeneous catalytic
toluene, cyclohexan, chloroform, dichloromethane system, the oxidation of further alcohols was carried
and also acetonitrile under irradiation at room out under the optimized conditions. The given results
temperature. Among the solvents examined, we inTable 2 show that this procedure was clean and all
found acetonitrile to be the best for this protocol the alcohols were converted into the corresponding
probably because of its photoinertness and all the carbonyl compounds in moderate to excellent yields
alcoholic substrates and their products were soluble without over-oxidation to carboxylic acids.
in this solvent.
Furthermore, this reaction is highly selective
For investigating the chemoselectivity, a forvicinaldiolsinoxidizingonlythesecondaryhydroxy
mixture of 1-phenylethanol and 2-phenylethanol was group ± to the benzene ring (Table 2, entries 15).

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AFSHARI et al., Orient. J. Chem., Vol. 29(4), 1675-1681 (2013)
Therefore, it is a method of choice for the oxidation
of benzylic OH in the presence of non-benzylic OH.
On the basis of previously reported mechanisms,
PTA in PTA/Si-imid@ Si-MNPs nanocomposite is
activated by light absorption and reacts with alcohol
to give the corresponding carbonyl compound. In
the presence of dissolved dioxygen in suspension,
the thermal oxidation of the reduced species of PTA
takes place at room temperature.
Compared with the other heterogeneous
2
8-30
POMs catalysts
, our catalyst (PTA/Si-imid@
Si-MNPs) has shown much more superiority. The
attractive features of this method are high yield, short
reaction time, clean reaction profiles, simple work-up
procedure, ease of separation, and recyclability of
the magnetic catalyst, as well as the ability to tolerate
a wide variety of substitutions in the reagents.
CONCLUSION
Easy and rapid separation of the catalyst
by magnetism is the most advantageous feature
of this catalyst. The recovery and reusability of the
catalyst was studied using 1-phenylethanol as model
substrate. Catalyst recycling experiments were
achieved by fixing the catalyst magnetically at the
bottom of the flask and the solution was decanted
after each run. The left solid was washed with
acetonitrile twice, and fresh substrate dissolved in
acetonitrile is introduced into the flask, allowing the
system to proceed for next run. It was found that the
developed catalyst could be used at least ten times
without any noticeable loss of its catalytic activity
In conclusion, a novel and efficient
procedure for the oxidation of primary and secondary
benzylic alcoholshas been developed using
immobilization of Phosphotungstic acid (PTA)
on imidazole-functionalized silica coated cobalt
ferrite nanoparticles as a green and recyclable
photocatalyst, in the presence of O as the sole
2
catalyst reoxidant. Moreover, the immobilized
PTA catalyst could be easily recovered by simple
magnetic decantation and reused at least five times
without significant loss of activity.
(
Table 2, entries 16). ICP-AES analysis has shown
ACKNOꢂLDEꢁMENTS
that leaching of the catalyst from support (0.8 % W)
occurred only in the first run and no leaching was
observed in the next runs. Furthermore, the FT-IR,
XRD and TEM (see Fig. 4) of the recovered catalyst
showed no change after using the catalyst for ten
times.
The authors wish to acknowledge the
support of this work by the Research Council
of Shoushtar Branch, Islamic Azad University,
Shoushtar, Iran
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